I. Field of the Invention
The present invention relates to a method for the continuous coating of the inside of an extruded hollow profile strand. The invention also relates to a device for removing excess coating agent from the chambers of a hollow profile.
II. Description of Related Art
For certain applications, such as for example the glazing of greenhouses or other humid enclosures, twin-wall sheets made of thermoplastic polymers, on the insides of which a water-spreading coating is applied, are used. For example, EP 0 530 617 A1 describes a method for the continuous coating of the inside of an extruded hollow profile made of thermoplastic polymer. In that method, directly after extrusion, a hollow profile strand is guided on a curved path through a supply of a liquid coating agent. After running through the coating agent, the twin-wall strand is guided upward until the entrained excess of liquid coating agent has partly run back into the supply.
One problem of this method is the slow run-off rate of the coating agent. As a result, more coating agent remains in the hollow chambers of the strand than is required for the formation of a uniform film on the inner walls. Such an excess leads to the formation of relatively thick and slow-drying films or else to the formation of flow edges, streaks and so-called “flow noses”. As a result, the sheets sawn from the hollow profile are wet.
Although the sawn sheets are treated at 60° C. in a conditioning oven for the purpose of applying an outer laminating film, this treatment is not adequate to remove excess coating agent. The previous solution was to place the sheet onto a carriage with running wheels, which tips the sheet in transverse and longitudinal directions, whereby some of the remaining liquid runs off.
Since the remaining residue of liquid is still very great, the sheets are subsequently connected to a hot-air fan, it being possible for the sheets to be dried individually in a discontinuous process. With the usual amounts of coating liquid, after this process crystalline deposits remain in the hollow profile, occurring as white spots, especially on both end faces of the sheet. To remove these remains, 300 mm must be sawn off on both sides of the sheet and form waste material.
Sawdust produced as a result must in turn be removed from the sheet. This step negates the advantages of the previously performed non-cutting operation of severing after scoring. It is also disadvantageous that remains of liquid or crystalline deposits get under the previously applied laminating film on the outer sides of the sheets and cause it to come away.
An amount of liquid coating agent inside the sheet in excess of the amount required for the formation of a uniform film therefore has the result that the continuous extrusion and coating process has to be followed by discontinuous, laborious reworking steps and that reject fabrication with 6-10% material wastage has to be accepted.
U.S. Pat. No. 5,681,390 describes a spray booth for the spray coating of objects, the inner walls of which are cleaned of finely distributed material by wiping bars. The wiping bars on the inner walls are moved from the outside by means of magnets.
Similar systems are used as magnetic window cleaners for aquariums. In those systems, a cleaning magnet on the inside is guided along the window by means of a magnet on the outside, whereby the inside is cleaned. However, the known techniques do not involve continuous methods. The principle of wiping off the contaminants is based on the idea that the wiper is moved while the location on the workpiece that is to be worked is stationary. Moreover, they are only suitable for the removal of solid remains and do not offer a solution for the removal and recovery of excess liquid remains.